1. Field of the Invention
The present invention relates to corrosion inhibition and more particularly to a new technique for inhibition of corrosion applicable to a wide variety of systems and metals.
2. Description of Prior Art
A solution has long been sought to the widespread and troublesome problem of corrosion. Corrosion has been an intractable problem with respect to various metal surfaces in a wide variety of systems. For example, corrosion of ferrous surfaces in oil refinery overhead streams, heat exchangers, towers and tower pump-around systems (in particular, of the crude distillation unit and vacuum distillation tower) and other distillation towers has not been solved to complete satisfaction. Likewise, corrosion of other metals and corrosion associated with other systems, whether such corrosion is oxygen corrosion, carbon dioxide corrosion, acid corrosion or otherwise, remains a serious problem.
An example of one particular situation in which oxygen or hydrogen sulfide corrosion is especially troublesome is refinery overheads. It has been difficult to solve the problem of corrosion in refinery overheads because such streams are highly acidic, typically having a pH of from less than 1 to about 3, and are maintained at temperatures exceeding about 200.degree. F. (93.degree. C.). By contrast, conventional corrosion inhibitors typically are employed in environments that are characterized by far less severe conditions. For example, corrosion inhibitors employed in oil field pipelines generally are not considered satisfactory corrosion inhibitors for refinery overhead streams and distillation towers, first because the disparate nature of the oil field pipeline and refinery/distillation technologies results in a failure to consider application of corrosion inhibitors from one art to the other art, but also because oil field pipeline fluids ordinarily are not strongly acidic (rarely, if ever, having a pH below about 4) and are at generally ambient temperatures. Thus, oil field corrosion inhibitors are not recognized as effective in highly acidic, high temperature conditions, which conditions themselves increase corrosion rates dramatically.
Accordingly, whereas the refinery and distillation streams include a strong acid, HCl, with which the corrosion therein is associated, and are maintained at a temperature of at least about 200.degree. F. (93.degree. C.), and often as high as 300.degree. F. (149.degree. C.) or more, oil field pipeline corrosion is associated with weak acids due to the presence of hydrogen sulfide and carbon dioxide and typical pipeline temperatures are under 100.degree. F. (38.degree. C.).
Because corrosion inhibitors have not been found to be satisfactory under the low pH, high temperature conditions of refinery overhead streams and distillation towers, it has been common practice to attempt to resolve at least the acidity problem by neutralizing the stream by addition of ammonia or certain organic amines, such as ethylene diamine, to raise the pH above 4 (generally to about 6) before addition of the corrosion inhibitor. This technique has been found to be unsatisfactory not only because of the extra treatment step and extra additive required, but also because the amines added to the stream tend to form corrosive HCl salts, which tend to exacerbate the problem and to corrode. Efforts to find suitable corrosion inhibitors for such applications typically have not produced entirely satisfactory results. Moreover, many inhibitors currently in use contain phosphorus, which affects catalysts downstream deleteriously.
According, while U.S. Pat. Nos. 4,332,967 and 4,393,026, both to Thompson et al., mention that the particular compounds disclosed therein might be applicable to refineries or distillation towers, corrosion inhibitors for oil field pipelines are not recognized to be applicable generally to refinery overhead streams, especially without first neutralizing the HCl in such streams. Thompson et al. also mentions (at co. 20, lines 29-33 of '967 and col. 20, lines 4-8 of '026) that the corrosion inhibitors described therein are effective in systems of "high temperature, high pressure and high acidity, particularly in deep wells, and most particular in deep gas wells." However, the acidity of such wells usually is not below about pH 3.5, generally not below pH 4, especially in wells that are not of high temperature and high carbon dioxide content. Thus, Thompson et al. do not suggest that the compositions described therein would be effective at lower pH's (as found in refinery overheads), or that their use in refineries would be in a manner other than the standard, conventional technique, which calls for addition of ammonia or an amine to increase the pH above 4 (with the problems connected therewith). And more generally, conventional corrosion inhibitors have been found to be either ineffective or susceptible to entering into undesirable side reactions in the highly acidic conditions of refinery overheads.
U.S. Pat. Nos. 4,770,906, 5,106,691 and 4,900,627 to Harwell and O'Rear mention in passing the possibility of the formation of a corrosion barrier, but disclose no more in that respect.
Thus, corrosion inhibitors that are effective in the low pH, high temperature conditions of refinery overhead streams without the need for neutralizing the HCl in such streams are needed.
Likewise, other corrosion inhibitors are system specific or not as effective as desired. Thus, corrosion inhibitors that can treat a wide variety of systems and that are even more effective at inhibiting corrosion are still being sought.